ROUGH SHIFT FROM 1ST. TO 2ND. WHEN VAN COLD

Chevrolet Venture

I have a 2ooo chevy venture van 3.4L engine with 115,000 miles. It has a delayed rough shift from 1st. To 2nd. When van is cold and the outsde temp. Is below 45 degrees. Problem goes away after 5 or 10 minutes of driving. All other shifts are smooth. The 1st. To 2nd. Shift is smooth if it's warm outside. Transmission fluid is in good shape. Any ideas what could cause this? Thank you.